You can camcreate a shell anywhere, it doesn't have to be at the vehicle position. If you wanted it off to one side you'd make the position something like this
shell1 = "shell125" camCreate [(getpos tank1 select 0) + 5, (getpos tank2 select 1) + 5, 0]
If you wanted the explosion to be in a particular place relative to the tank you'd need to use getDir and some trigonometry. Look in the weapons and ammo listings in the Ed Depot for alll the explosion types you can have, experiment till you get what you want.
If you want a delay you can easily put one in a script
~3
or trigger using the countdown field. If you want random positioning of the explosion you could do something like this in a script
_rand1 = random 10
_rand2 = random 10
shell1 = "shell125" camCreate [(getpos tank1 select 0) + _rand1, (getpos tank2 select 1) + _rand2, 0]