There are two schools of thought as far as I am aware:

1) get multiples of the same thing - so you can learn one thing inside out and use it multiple times - very cool approach - it works!

2) get different flavours of essentially the same thing, so 1 Maths and 1 Falistri or some other variant - also very cool and works well!

I have a mix of both approaches, so 2 Stages makes loads of sense because they can be chained to increase the functionality, but my other function generators are all different, Terminal, Maths, Double Andore (mkI and mkII), Tides, Zadar - each having it's own special flavour and tricks!

Ultimately I think it comes down to what you want to achieve :)