Is this a girder beam where the beam sits on top of the posts and the joist sit on top of the beam and/or carry past? or, Is this beam also the outer box frame (flush framed) where the joists just butt into it? Also, it would help to know if the posts have to carry past and be supports for the railing above. Personally, I think 4x4's might be slightly undersized and 4x6's might work better for a deck this size.
I thought you were talking about splicing the 2x beams together but by "splicing" I think you are talking about the connection between the two 2x's beam and the posts. If the posts are part of the double outside "beam" box and you want to run them up through to support the railings, then you can just bolt or lag the band beam to them or hog out 1" pocket for the first 2x to rest in and then lag it home. If you have 6 posts for a 27' beam supporting a 12' wide deck then you aren't really talking about any huge loads. You will barely have 5' spans so your loads are pretty well spread out. Post another message with a few more details if this doesn't help.