Is it true.. that when you replace a shock (Front/Used) that you should replace both?

The advice was to replace shock absorbers is axle pair. Iow not to have one new shock on one side and an old shock on the other. I believe the idea was to avoid imbalance, potentially leading to unsafe handling.

I'd try to get a pair of used shocks from another car and use both. Otherwise I'd do the old 'bounce test' after fitting one used shock, as in, trying bouncing the car at each corner several times to get a rythm going (like pushing someone on a swing), then stop and see how long it takes for the car to stop bouncing. It should stop after less than one up-and -down cycle (iirc). Better than nothing, I suppose. Or have someone observe the suspension damping action of both front? wheels as you drive past on a slightly bumpy road to see if everything looks normal.
You could always take the car to a garage or tire center and have them check the shocks on their equipment. (If you like spending money, I don't
).

Al.