I saw many videos about how to using 2 mould to create stuff, usually lay composite on each mould separately, while the problem is when people are doing this, the joint place got no composite connected, they are only glued together, how to strength that part I am wondering, I guess this is a very common questions most people will encounter. Let's discuss it a little bit, how to solve it? if not possible or too hard, how to bypass it? Thanks.

Usually when you are doing a split mould part you either do the first ply net to the edge or you overlap on one side by at least 10mm.
You then stagger the joint either butt joints or overlap to ensure that you don't get the chimney effect and an overall poor laminate.
What part are you looking to make?

Thanks, Scott.

I am looking to do a bent pipe with manifold. I guess the best way is to split to two, however, feel it is hard keep the shape inside and strength at the joint place.

Actually, I want the inside surface of pipe to be smooth too, if I put a mould inside, how can I remove the inside mould? If I don't have a inside mould,  how can I be sure the 10mm margin stick to the other side, using finger or any tool to stroke is not precise. So, actually, many questions here. Don't have a clue.

is the paraffin strong enough to be used as a mould? if it is, I gonna wrap it with fabric and melt it afterwards, how is that?

There's a few options when doing pipes and manifolds, internal bags, intensifiers and soluble Mandrels. 
I would seriously do some research before you commit to this.