Back in the 70s some guys put 1716s, I think it was, inside 2020s and got finished arrows at around 1000 grains. Those arrows could penetrate a car door.
2219 with a 1918 inside would be pushing the 1000 grain mark too.
I have to ask why do you want your arrows heavier? They should be just under 700 grains and that should be plenty heavy enough.
I made an insert and arrow water tight once and filled the shaft with water. The arrow was really heavy, sorry no grain scale at the time, and hit really hard but didn't fly well. A teenagers experiment gone awry.