How to hook up 2 batteries in parallel ?

I'm building a portable MP3 "boom box". I have two 12 V SLA batteries on hand, one is 12 V 4 AH and the other is 12 V 7 AH capacity. My question is: can these 2 batteries be connected, in parallel to power the amplifier I have ( 12 V) and
1) it would give me a ~ 11 AH battery?
2) could I charge them both, ( connected like that ) with a 12 V battery charger ( a automotive battery trickle charger) I have at hand?

Yes, you can parallel them. In general, unless the batteries are new, its not a good idea, because one will tend to hog the charging current, and provide most of the discharge current.