Given you're likely replacing the bikes anyway, I had a google and found that a rough standard'ish capacity for a new electric bike is of the order 500Wh. At 12v, that equates to about 42Ah, which when doubled for two such bikes gives you 84Ah. Now, we can go through it in more detail but it's clear that this is going to require a very significant battery investment to be feasible (having said that, it is probably at least possible from a pure battery perspective - search the forum for a 200Ah 'Roamer' lithium battery that showed up recently). However, from a solar perspective, if we assume that, at a worst case, you'll be charging these bikes daily, you're approaching the limit of what solar can reasonably provide over the course of a (sunny!) day. If we assume a 300w panel (close to the largest size that can be reasonably accommodated), then on a day of near perfect sunshine for, say, 6 hours you can expect a maximum of 150Ah in but this will drop off extremely quickly as conditions become less than perfect and various inefficiencies in the system will detract further.
A further factor is that I don't know how widely available 12v bike chargers are. If one isn't available then you are going to need a pretty beefy inverter too.
With all this in mind, and before you spec out a >£2k monster electrical install, are you sure you need this capability off hook up? If you only need it on hook up then you're away, you simply plug your charger into the 240v socket...
Alternatively, if the bikes charge quickly, maybe you're happy running the engine to charge the bikes?
