If the ruling of a Mujtahid goes against the ruling of the jurist guardian, which one must be followed?

The ruling of the guardian jurist is the one that should be followed in issues that relate to the management of the Muslim nation and the general affairs of the Muslims. As for issues which are purely individual, it is possible for every person to follow their own Marja’.
To be more precise, it is obligatory upon all, whether they be a lay person or a Mujtahid, to follow the rules of governance that emanate from the jurist guardian. An example of this from the narrations is the ruling of the Imam or the ruler about the visibility of the crescent moon.
In the authentic narration of Muhammad ibn Qays from Abu Ja’far (Imam al-Baqir a.s): “if two people witness in the presence of the Imam that they saw the crescent thirty days ago then the Imam should order people to break their fast” (Wasailul Shi’a) Note, that the term ‘Imam’ in these narrations means ‘ruler’ (whether an infallible one or not), as discussed in the second chapter.
If there are Multiple Islamic Countries, should there be one guardian jurist for all, or is it permissible for each country to have its own guardian jurist?
The textual and intellectual principles indicate by necessity that the entire Muslim community must have one leader who unites them and links them together and rules over them in issues which require general rulings. It should be such that these countries should be considered one powerful government, with one country able to help another. The presence of multiple rulers who are independent in their opinion and will in all affairs, without having one leader to unite them and judge with certainty in their disputes is a sure way to disunity and failure.
Sheikh al-Saduq has narrated from Imam al-Ridha (as): “If he says: ‘why is it not possible for there to be more than one Imam on earth simultaneously?’ It would be said: ‘for a number of reasons, among which are the following. The actions and plans of one person cannot conflict, whereas the actions and plans of two people never agree.
We never find any two people except that they have different intentions and wishes. If they were two and their intentions, wishes and plans were different, and it was obligatory to obey both, then neither would be more worthy of being obeyed than the other. This would lead to disagreement, conflict and transgression among people. No one would be in obedience of one leader without being disobedient to the other, and thus all the inhabitants of the land would be sinners…” [115]
This is with the assumption that this is possible. If however we assume that it is impossible to establish one Muslim nation which encompasses all Muslims, then there is no problem with establishing small countries based on the laws of Islam. This is better than ignoring the affairs of governance until the foreigners and tyrants become rulers over the Muslims and overpower them. This (i.e. the establishment of different Islamic countries) would become valid because of the principle ‘that which is possible should not be ignored because of that which is impossible’.
Sheikh Mansour Leghaei
Source: Imam reza network