The common type acrylic plastic is a thermoplastic and melts at 160C(320F). That's used for display cases, domes, etc that are formed from sheets. It is also used in injection moulded objects.
But "Acrylic" is now used to describe a range of different plastics. Many of them are thermoset plastics. Thermosets don't melt, they burn and/or crumble when heated.
I think most acrylic pen blanks are thermoset plastics, like Polyester Resin (PR). You can not melt these together.
Even if you have acrylic blanks that will melt it would be very dangerous to try melting and pouring into a mould. Even molten you will have a very thick, viscous liquid that won't pour well and will probably solidify before it gets into the mould. That's why it is injection moulded. Also you have toxic fumes, the possibility of fire, and hot, sticky,molten plastic on skin.
You're better off gluing blanks together as others have suggested. Or if you want to make your own, use PR or Alumilite and cast it into a mould. You can then add your own colours and make custom blanks. There are plenty of threads and library articles on the subject.