I hope this isn't a silly question but here goes.
I have a 100Ah 900CCA starter battery connected in parallel (manual in-out switch) with a 115 Ah leisure battery.
If I start the engine with the two batteries connected, is there any possibilitry of causing any damage/doing any harm to the leisure battery?
There are a couple of issues here. If you connect two different batteries then the battery with the least charge/lowest voltage will pull from the other one, so generally always a bad plan to connect two dissimilar batteries as you could end up with two insufficiently charged batteries.
Think of two identical buckets, then take a pipe fitting out of each bucket at the same height, now fill the buckets full. If you take 3 cups out of bucket A, what will actually happen is you take 1.5 cups out of each bucket. If you used a water tank and a bucket and linked them the bucket is always going to end up having more as a percentage of the total taken out than the tank then the tank will refill. Also, the alternator will just fire current out to charge, but one of the batteries, unless a matched set is going to be hit too hard, or one not hard enough to charge it properly.
Aside from that being a problem, is the leisure battery strictly deep cycle use only? If so then having it connected when the starter kicks in will subject it to a large load which it was not designed for. What I think you should do here is either, setup a true and proper split charge with changeover so you can run the vehicle electrics on the leisure with the engine off and charge it via the split charge, which would normally only kick in once the main battery is full, or fit one big starter capable leisure battery or two smaller leisure battery if you don't want all your eggs in once basket - it depends what you do.
Personally I would never connect the two unless in an emergency where I maybe needed to use the leisure to start the engine, but at that I would want the dead battery well out of the way as it would start sapping a lot of power to try and recharge it, and I would probably make it so I had to move the battery cables to do this and there was no "switch".
In some sort of expedition situation, I would probably just carry a second battery in the battery box, disconnected and switch them round every so often to keep it charged if the temperatures were warm, or have two deep cycle start capable batteries (i.e. Optima Red top) each on their own capable of starting the LR but linked in parallel so they were never really given much loading and if one failed I would still have the second, however, issue here is if one fails, the good one will try and charge it, so back to why you'd want separation.
