i will do my first infusion on more complex mold. mirror cap
it is a split mold.

so question is: should i use envelope bag or not.
if not, how can i seal the split?

Envelope bag is a no brainer for such a small part.

You can seal the edges using gum tape, any bleed through only going as far as the gum tape.  Some people prefer to smear silcione sealant before joining the halves together.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant