"May add" house fortunes and head for numbers

[Pytorb]

Stannis also had Pylos and Cressen as maesters

Oops, yes he did...

There is no reason why one should have two maesters for any amount of time (apart from a 'change of the guards') but then there is no reason why one shouldn't feed and cloth two if one wants.
One the holding side i would cap the benefits though, meaning one maester is as good as two when it comes to house fortunes.

Personally I'd go the other way round, allow the benefit of two Maesters if the Lord wanted it, but say that the second Maester costs double due to all the extra persuading needed to encourage The Citadel to send two Maesters to one house. But either way the law of diminishing returns applies

[Zorbeltuss]

Pylos was sent to Dragonstone because Cressen was getting old. Which remains the only example of multiple maesters serving the same castle, it seems clear enough that a maester is supposed to serve whoever is ruling the castle they are assigned to, rather than a house. Which does explain why the Night's Watch has more than one maester. If a House has two castles, it could be possible that it has two maesters, though I would expect that to be somewhat rare.

Frenken is Stokeworth's maester, and Ballabar is Redwyne's maester. The Iron Throne have only had Pycelle serving it directly during the books. Though they certainly borrowed others, especially Frenken while Pycelle was in the black cells.

[Daerys]

Stannis is one of the most important lord of westeros, multiple maester at Knight Landing is not weird, this is the heart of the power in westeros.

Two maester in a minor house is a waste of resource, it's also why the house must have an influence of 20 or more to buy this holding, you should allow two maesters but ask for an influence of 50 or more...