My ideal Starbound would have custom items/blocks. By that I mean you can dismantle tools and weapons and get parts back. You can merge these and upgrade the stats. For example, if you merge a RPG and a pistol you will get a gun with a primary fire of a gun and secondary fire of a RPG. What would your merge be and what do you think?
I think dismantling tools would be great! You could collect scrap parts and build it into a low level gun, the better parts you have the better the gun! Although I don't know about merging items, it seems overpowered and not really useful.
For what the OP is talking about, I have to tell you that it doesn't seem like secondary fire is supported. In fact, you could achieve the same affect by having a pistol in one hand and RPG in the other. Then again, if there are two handed weapons...
Yeah I think it's a good idea that you could use items you don't want for another thing than just gold, since you can only sell them. But I agree with Woodledude, you shouldn't be able to merge them, since you can just take them each in one hand.
And why would you deconstruct something except to reuse the parts to begin with? Item combination is a good idea on its own though.