Sounds like you are talking about them being wired in series, for 12 volt
function. So a single would need to be a 12 volt unit., at 5 amps.
Perhaps such as at http://store.yahoo.com/batterysupplyco/index.html

oo Mike oo wrote in message
...
I have two 6-volt, 2.7AH batteries, connected together to my alarm
system. (Batteries are connected to each other by a wire on one end,
connected to the system by the other end)
It seems expensive to get two of these batteries at about $20 each.
Can I replace these with a single battery of the same type or do I
need to buy 2?
Would a single 6-volt 5.4AH do the same thing?

Thanks in advance,
Mike