Small sample size, but I think the best way to do this is as follow.
Get the team that has the two Players you are looking for. Similair ID, big gap in SB. Now, schedule weekly your Scrimmage against this team. Same tactic every week. Difference is, one week the SB guy is at C, the next you swap him with the other.
Now some things will still change as any training pops on either team will affect this, and Game Shape/Stamina will still be in play. But Ability won't change DRASTICALLY week to week, and you'll get to face the same C each time.
Note: Even a Pop in Passing for the other team, or OD for your team, will affect this. Why? Passing improves the Shot, so now the C you play against will have a better shot than the week before, and OD means more pressure on passes so the C you play against will (A) not get the ball as much or (B) will get it but in a worse position to shoot than previously. Seriously, ANY pops on either team will change things.
Still, if you can get some one to make you the Scrimmage every week and assure that all the Settings are the same, you can swap your C and PF to 'test' SB and Help Defense.