Magnetic force and Coulomb force
If you have just 2 electrons moving there is no point in saying "less electrons now fit the contracted space" There will still be exactly two electrons no matter what frame of reference you are using. 

I guess what you have in mind (still some mind reading required with your question) is looking at two parallel wires with a given current (which is reflected by a given charge density (charge per length) and speed) when observed from rest and from a frame of reference moving with the electrons.
There will be difference in Coulomb force because of differences in charge density due to relativistic effects (length contraction) depending on the frame of reference; and the difference will exactly match the difference of magnetic force which will be present only when observed from a frame of reference not moving with respect to the electrons.

This is a well known illustration of the fact that magnetic force is just a relativistic effect of electric force.

You can find it discussed e.g. in Feynman Lectures of Physics, Volume II, "Mainly Electromagnetism and Matter": 13–6The relativity of magnetic and electric fields