If body a and b are both within system can forces between them affect the acceleration of the system?

Accelerating a system would change the momentum of the system.

Momentum is conserved until there is an external force on a system.

Internal forces do not change the momentum of a system because forces come in pairs and they are equal and opposite to each other (Newton's 3rd law), so all internal forces cancel each other out.

Internal forces can't accelerate the center of mass of a system.

It affects the muscular and digestive