Latin Catholics are allowed to attend the SSPX. I see nothing wrong with “Orthodox in Communion with Rome” attending an Orthodox Divine Liturgy. It is crazy to think the Orthodox Divine Liturgy ceases to fulfill my obligation just because I am in Communion with Rome.
An obligation is an ecclesiastic binding by those put over us who say this is the minimum we’ve decided you need to do in order to not lose your salvation. They get to make the rules, like which parishes you’re allowed to attend, when they set out those minimums. If it isn’t a sin they bind on you, you have to be obedient, even if you think it is crazy. You don’t get to make your own rules because you think you know better.
The Vatican used to be wishy-washy on the status of the SSPX and of the laity who attend there. A 2003 letter from the Pontifical Commission “Ecclesia Dei” under Dario Cardinal Castrillon Hoyos allowed attendance “in a strict sense” to fulfill the obligation, but strongly said they “cannot recommend your attendance at such a Mass” and warned of the possibility of sin in doing so if it was coming from a “desire to separate yourself from communion with the Roman Pontiff and those in communion with him.”
A lot happened in SSPX relations between now and then. Their position and the relationship they have with the Church is clearer. The directions given to the laity have changed, too. The bishops get to change the man-made rules they bind on people in order to address the circumstances and needs as they change. Sometimes they loosen and sometimes they bind. Regarding the SSPX, they responded to the changes in the relationship by binding those loyal to the Church.
In February 2012, William Joseph Cardinal Levada, the President of the Pontifical Commission “Ecclesia Dei,” answered a question about whether a Catholic may fulfill his obligation on a day of precept by attending an SSPX or similar Mass. In an apparent reversal of the commission’s guidance from a decade before, he made it clear that it is not a sin to attend SSPX or related Masses, but they cannot be used to fulfill a Roman Catholic’s obligation.
In November 2012, the same body received the same question with the qualifier that the SSPX Mass was “the only opportunity in the local area to participate in the Mass in
forma extraordinaria (which the participant is highly devoted to).” They gave the same response, using Pope Benedict’s words to reiterate that “the Society has no canonical status in the Church, and it’s ministers – even though they have been freed of the ecclesiastical penalty – do not legitimately exercise any ministry in the Church.”
Individuals who attend SSPX Masses in place of a canonical Mass sin gravely because they do not attend Mass at a Catholic Church on the bare minimum of days. Sacraments requiring licit orders, including confession and marriage, are invalid while the rest remain illicit. Laity are excommunicated
latae senteniae if they attend SSPX Masses out of “withdrawal of submission to the Supreme Pontiff or from communion with the members of the Church subject to him.” Neither are devotion to the liturgy or paucity of opportunities to attend the liturgy of one’s preference valid reasons to avoid minimum obligations.
If you’re a Roman Catholic, accept all it’s rules and live by the minimum standards. If you disagree with them, petition the person who can release you from the minimum obligation. Or be honest and acknowledge that you aren’t following the Church. Don’t keep a shoe in one, two, three, four doors and try to convince others it is good for you or the Church to pick and choose. The commission is clear that it does not recommend this course of action as it is at the minimum an occasion of sin, frequently a mortal sin, and opportunity for full excommunication.
Eastern Catholics and their relationships with the Orthodox are different because different disputes are had, different agreements reached and different governing bodies and rules are involved. To quote Roman Catholic canons and requirements and try to apply it to Eastern Catholics hurts all involved by muddying the waters. Roman Catholics have to follow their minimum obligations and the guidance their hierarchy gives. Eastern Catholics have to follow their minimum obligations and the guidance their hierarchy gives. The hierarchy has the privilege and responsibility of making those rules for their people. There’s no middle ground or cherry picking allowed. If you want to know what you should do, the only option is to get a meeting with your bishop or patriarch or their representative and ask.
I meant that the very worst some may consider them schismatic since they don’t teach heresy and therefore cannot be called heretics. 
Since the OP is a Latin Catholic, I answered it from the General Western view.