I've searched and searched to find the following suggestions:

1. Adjust the valves (twin cyl, no ohv, not sure this engine has valve adjustments)
2. ACR broken (still trying to figure out if this has ACR or not)

Note: According the B&S IPL for this engine, there is no visible adjustment for the valves nor is there a compression release (when searched by name, compression is not found in the IPL document).

The mower belongs to my cousin and has been sitting for about 2 years. Rather than have it sit outside for an additional 10 years, I was hoping to fix it so they don't have to keep borrowing my mower. Enough backstory?

Rider: MTD
Motor Briggs & Stratton 16.5 Twin Cyl
Model: 42a707
Type: 2238 e1

-The motor will turn and start on "either" cylinder if the plug is removed from the opposite cylinder.
-Motor stops at compression stroke when both plugs are installed. (jumper cables from running vehicle to ensure power)
-Motor spins free when no plugs installed (duh!)
- removed both heads to verify there were no "stuck valves". (all "looks" well)

Feeling as if my head is against a brick wall (I've rebuilt chansaw motors, but never touched a B&S small engine before), I would like to ask for direction in finding/fixing this issue.

Thoughts?

Resolved by draining the crank case. Engine is now able to start.

Hey skellum are you saying all you did was change the oil and it started? I have the same motor new starter just not enough power to crank even when on jumpers. Did you find if they have a decompression valve? And you are right the valves have no adjustment.

The way you do the valve clearance is take them out and grind the ends until proper clearance is made.
And it does have a compression relief on the camgear.