Je ne pense pas que… je sois le mieux placé sur FSE pour recommander l’usage du subjonctif quand… on ne veut pas en mettre! 🙂

Voilà! Si j’ai bien compris la question, on peut toujours utiliser le subjonctif avec les verbes déclaratifs, d’opinion, de croyance… même en cas d’identité des sujets.

On peut! Aucune sorte d’obligation donc, j’aurais aussi très bien pu dire que …

Je ne pense pas… être le mieux placé. Comme je l’aurais certainement préféré écrire.

It is possible to have a subjonctive subclause whose subject is non-obviate to that of the main clause, but most of the time this is restricted to complement clauses introduced by a preposition such as pour que or avant que that are obligatorily used with a verb in the subjunctive.

J¹’en ai eu assez bien avant que je¹ ne parte

Estelle¹ avait tout prévu pour qu’elle¹ ait le voyage le plus rapide possible

In fact, the sentence in the question is an example of one such occurrence, since "à condition que" requires the subjunctive in contemporary usage (you can find it used with the indicative in classical French, but this has gone out of use).

With direct subordinate clauses, this is much rarer, especially in writing where good style would lead most writers to correct any such sentence. Despite that they occasionally occur in speech and you can find some written exemples, often but not necessarily imitations of spoken style:

Elle a fait non de la tête lorsque j’ai suggéré que je la ramène à son appartement (from a translation of Tom Holt’s The Candyman)

Laisses moi être ce que je veux que je sois (sic, from this webpage, probably trying to avoid repeating être)

Syntacticiens (references at the bottom of the answer) trying to judge the acceptability of this type of sentences have found several factors that facilitate the appearance of a subjunctive instead of the expected infinite:

1. Lack of agency of the subject: "Je préférerais que je parte à l’heure"

2. Passive subclauses: "Je veux que je sois autorisé à partir tôt"

3. Perfectivity of the action: "Je veux que j’aie le temps de me préparer à partir"

4. Coordination between two different subjects: "Je veux que tu partes et que je mange" (somewhat less acceptable if ordered as "Je veux que je mange et que tu partes")

5. Distance between the two verbs: "Je voulais à l’époque absolument que je sois le premier"

This would suggest that a weakening of the link between the two verbs as sharing the same agent tends to induce sporadic appearances of the subjunctive. The same factors have also been linked to the (again, sporadic) appearance of the indicative where a subjunctive would have been expected.

I’ve ignored cases with impersonal subjects like "il faut qu’il parte" or "il est possible qu’il fasse mauvais demain" since both subjects are different, even if they share a grammatical person.

FARKAS Donka, "On obviation" in Lexical matters, 1992

KEMPCHINSKY Paula, "What can the subjunctive disjoint reference effect tell us about the subjunctive?" in Lingua, 2009

POPLACK Shana, "Prescription, intuition et usage : le subjonctif français et la variabilité inhérente", 1990

RUWET NIcolas, "Syntax and Human Experience" (chapter 1), 1991