Under What Conditions Can a Priest Validly Refuse to Absolve Sins? Absolution can be withheld, I was wondering under what conditions such an action can be considered to be valid(not licit or illicit), if any are needed at all. Furthermore I want to know if a priest can validly absolve the sins of someone who has been refused absolution from another priest validly.
Under What Conditions Can a Priest Validly Refuse to Absolve Sins?


  The denial (or delay) of absolution is rare in a typical confession. When it does occur, it usually is because the confessor is not able to observe sufficient contrition from the penitent. As the Catechism teaches: “Contrition is sorrow of the soul and detestation for the sin committed, together with the resolution not to sin again” (No. 1451). So, mere sorrow is not enough; there must be sufficient resolution to avoid the sin in the future. 
  
  A  confessor must do his best to dispose a penitent for absolution. Usually sorrow for sin and a firm purpose of amendment are evident in someone who has sought confession. However, in some cases, a penitent may give no signs of sorrow or may show no resolution to avoid obvious or near occasions of sin. Perhaps a penitent indicates a refusal to restore what is possible to rightful owners. Or a penitent who does not indicate a willingness to end illicit sexual unions or practices. Though they may experience sorrow or feelings of guilt, they cannot or will not supply a resolve to avoid the sin in the future. 
  
  The priest should do what he can to draw both sorrow and a firm purpose of amendment. But if there is no evidence of these, he should deny or delay absolution in a kind and fatherly manner, explaining the reason and also a way forward so that the penitent can return more properly disposed to receive the sacrament. 
  
  Generally, the simple fact that a penitent has sought confession is a demonstration of contrition and amendment. But there are rare times when during the confession something essential regarding contrition is found to be lacking. To simply overlook this does harm both to the sacrament and to the proper care of souls. - Refusing absolution 


Thus a priest must refuse absolution if no contrition is present or no willingness to amend one's life of serious sin is present. One not willing to amend a life of adultery for example can not be absolved in confession. Absolution in this case would be invalid.

Another case may be when the penitent is under excommunication.


  Finally, there many possible reasons why Catholic may be "prohibited by Law for receiving" a sacrament. If, for example, he is under excommunication, he is forbidden to receive the sacraments until the excommunication is lifted (c. 1331.1 n.2). He may simply be too young to receive a particular sacrament. - When can a priest refuse to absolve a penitent in the confessional?


Confession while under the sentence of excommunication would not be valid and a priest must refuse absolution in some cases, unless one is in danger of death.


  In some very grievous cases, only the Holy See can lift the ban of an automatic excommunication:  if a person desecrates the Blessed Sacrament or uses it for a sacrilegious purpose (#1367); if a person uses physical force against the Pope (#1370); if a priest absolves an accomplice in a sin against the Sixth Commandment (#1378); if a bishop consecrates someone as a bishop without permission of the Holy Father (#1982); and if a priest directly violates the seal of confession (#1388). - Automatic Excommunication


As for the sacrament of the Orthodox Church this should help:


  “If attending was done in the spirit of ecumenism, would it be ok as long as I still went to Catholic Mass as well?”
  
  Yes, and the obligation to assist at a Catholic liturgy is not fulfilled by going with the Orthodox family to their church:
  
  Can. 1248 §1. A person who assists at a Mass celebrated anywhere in a Catholic rite either on the feast day itself or in the evening of the preceding day satisfies the obligation of participating in the Mass.
  
  Can. 844 §2. Whenever necessity requires it or true spiritual advantage suggests it, and provided that danger of error or of indifferentism is avoided, the Christian faithful for whom it is physically or morally impossible to approach a Catholic minister are permitted to receive the sacraments of penance, Eucharist, and anointing of the sick from non-Catholic ministers in whose Churches these sacraments are valid.
  
  Footnote: In regard to canon 1248, an argument could be made that an Orthodox liturgy is “a catholic rite” since the Church herself acknowledges that Orthodox (both Eastern and Oriental, plus ACoE) Sacraments are valid. - Is it valid for a Catholic to attend an Orthodox Church service?


Do not forget that both Pope Paul VI and Patriarch Athenagoras I signed the Catholic–Orthodox Joint Declaration of 1965 


  The Catholic–Orthodox Joint Declaration of 1965 was read out on 7 December 1965 simultaneously at a public meeting of the Second Vatican Council in Rome and at a special ceremony in Istanbul. It withdrew the exchange of excommunications between prominent ecclesiastics in the Holy See and the Ecumenical Patriarchate of Constantinople, commonly known as the Great Schism of 1054. It did not end the schism but showed a desire for greater reconciliation between the two churches, represented by Pope Paul VI and Ecumenical Patriarch Athenagoras I. The document and accompanying texts are also referred to as 'Tomos Agapes' ('Document of Love') - Catholic–Orthodox Joint Declaration of 1965 (Wikipedia)


In the end a priest can validly absolve any penitent who has been formerly refused absolution (validly) provided that all the requites are in place to have a valid confession.