I've seen several systems for tech sharing that involve points or quotas on the number of techs allowed. However I it would be simplest to make the cost of tech sharing a discounted function of the original cost in time and ics. Maybe make shared techs cost 2/3 of the original tech in time and ics. Thus a 10 IC, 150 day tech would cost 6.6 ics and 100 days to share. This cost would both limit the overall number of techs shared, and discourage say Germany from sharing minor techs like "Improved Vehicle Radios" with Spain. In addition, if you could only share one tech at a time, this would mean yearly 2-4 techs would be shared, and the Allies/Germany would find it impossible to make Russia/Italy respectively technologically equal by 1939.
Besides realism, the principle advantage of this method would be it uses our existing tech system, rather than having to introduce a new one.