Would you use a primary fuse for each transformer or a single (bigger) fuse would do for both? I'm leaning towards to using one fuse for each, it seems safer that way...
On the other hand, I've read it's advisable to put mains fuses before the switch.
Now if I chose to have a fuse for each transformer, I would have to have two switches, which just doesn't sounds that great
A single fuse can protect you only if a fault on either of the loads will draw enough current to blow it. If the transformers are similar, this should not be a problem.
I would suggest it depends what you're doing?, as you don't say - if you're simply paralleling two transformers then use a single fuse - if it's two completely separate and different transformers, then use separate fuses.
if you're using separate transformers of different sizes (i.e. one for a low current "standby" supply, and one for a power amp) use separate fuses. if one is a low current standby supply use a fast-blow (type F) fuse. if one is a supply with a large inrush current (like for a power amp or a switching supply), use a slow-blow fuse (type T). you may want to even have a third fuse before the switch as an overall line fuse. it's not unknown for a switch to develop a short on it's own or some other component between the AC line and the transformer short either internally or to the chassis. the line fuse is usually in a panel mounted fuse holder rather than on a board inside the unit.