I would send them out about one month prior to leaving for Jamaica. That way you can try and have most of the rsvps back before you go and you will have less to do when you get back.
My sister is doing a 2 in 1 invitation- info for both the mexico wedding and the at home reception. she is then going to have two reply cards so that pple can let her know if they plan on attending 1, both or neither.