Space Marine Attack Bike Twin-Linked bolter
On a bike you can fire as many weapons as there are riders.  So an attack bike can fire up to two weapons since it has two riders.  This is regardless of how-many wounds the bike has sustained. 

The entry for the the Space Maine bike in the equipment list will indicate which weapons it has.  Much like the entry for Terminator Armor indicates that it allows deep-striking. The twin linked bolter is mentioned there.

And there is no good reason to shoot anything but the heavy weapon and the twin linked bolter.  You suffer no penalties for using them on a bike and are hands down a better option than the pistol in every way.