Yes you can get a reasonable battery for around the £50 mark.
If you mean the physical location 2 batteries should fit in the battery box Just make sure you don't make it airtight!!!
If you mean the wiring side of it there are plenty of kits you can buy or there are plenty of threads explaining how to do it yourself.
it is generally accepted that yu wont get two batteries in the same condition, and it will mean that one will pull the other one down - that is why split charge systems disconnect the secondary battery when not being charged.