Have you owned this boat since new?
As you did mention someone had taken your trim switch apart?

Are your twins side by side, as in a single hull/ vee hull, cables usually run in same chase way under deck, or split as on a cat, cables could run in two separate chase ways, twin hulls? If a previous owner opened trim switch, he may have pulled entire controls, if replacing cables, or checking wiring on that switch? Could he have damaged/kinked shift cable on SB side, looking for problem?

A possible quick test if cables, disconnect at motors, should have easy quick-disconnects, check shift and throttles on each motor, if smooth and no resistance, probably good. If SB has more resistance than port, could have damaged cable, or friction on control set too tight?

Have you inspected/ changed gear lube for both lower units yourself recently, could tell a story?

As stated, i don't know if you're the original owner, but you mentioned someone was in the switch, and mentioned cables were just a couple/few years old.

Just thoughts, good luck.

Just got the boat. He said he did cables last year and it has become stiff. The throttle portion is easy, it's the shifting that's labored.

Clutch shaft bushing is stove up. Not a huge deal to replace, just trace the shift linkage to the arm that moves back and forth. You will need to drop the LU. From there, it's 2 bolts to remove the shifter mechanism. You'll have to do some twisting and turning but it will come out on the port side. The nylon bushing has probably become 1 with the sleeve it sits in. Part # 16 on the attached diagram is your likely culprit.

2003 DF140 Suzuki Marine Outboard CLUTCH SHAFT Diagram and Parts

Forgive me here, I have not messed with these engines yet.

Lower unit just unbolts at lower, nothing up top to unbolt?

What panels must be removed to gain access to that shaft?

naturally, this is the SB engine and these guys are close together. thinking I might have to pull the tie bar to allow it to swing away.

Yes, the LU just unbolts from the mid-section. You'll need to remove the small trim tab right above the prop to access the last bolt. You'll only need to drop the LU a couple of inches, just enough for the clutch shaft to drop out of the holder. Rest it on a block of wood. You'll need to remove all the cowlings, top, sides and front.
Disconnect the shifter cable, drop the LU and try to move the clutch lever by hand. If it's really stiff, that bushing is your problem. I had twin 03 DF140s and had the exact same issues. Really tough to shift but throttle was easy. Replaced the bushing and greased it up good, no more problems.

It appears that my area of tightness is at the shifter linkage on the side of the engine. The part that has the zerk and shift cutout switch. cable action is easy as is shift rod that goes down to lower unit. Per parts diagram, clutch lever holder part #2