correcting delay between two samples I'm sampling two signals with fundamental frequency of 50 Hz. I want to sample them at the same instant, but there's a delay between the first and the second conversion. I don't keep the previous values of the signals in an array, at most I can store 10/20 samples.

I want to correct this delay. I thought of a FIR fractional delay, but i'm not sure is the right way to do it.
Assuming you are sampling above the Nyquist rate, then an interpolation kernel or equivalent fractional FIR filter on one or both channels (both to match up any non-zero processing delays) might be an appropriate solution.