I can only address the FAA rules applying to type ratings.
In FAA land, it's done allot in the corporate and charter business aircraft sector. I maintained PIC currency in two types concurrently for several years. For part 91 ops, the requirements of 61.58 stipulate that a PIC proficiency check must have been completed within the preceding 12 calendar months and that this check must have been accomplished in the specific type within the preceding 24 calendar months. So if you do a PC in one of the types annually and alternate between types each year, you satisfy the requirements of the reg.
For part 135 ops, just cut the time interval in half. 135.293 and .297 can be satisfied by completing training/checking events every 6 months in alternating types. I went every 6 months and alternated between the two types I was flying. The two types I was flying were not very similar, but remembering which airplane I was flying at the time wasn't a serious problem.
The rules don't specifically limit this practice to two types either. A few pilots at one company I worked for maintained currency in three types to maintain 135 currency. (G-1159, G-IV and G-V) Those guys went to the sim every 4 months. Given the commonalities between these types, it wasn't a big deal switching between types for them.
The safety auditing firms Wyvern and ARG/US had a policy of limiting their approval to two types concurrently though. Having these private firms approval is necessary in order for part 135 operators to serve clientele subscribing to their services. And apparently the FAA adopted this policy for 135 operators also, but not through the rulemaking process. More of a policy rather than a rule. There might be an inspector handbook bulletin somewhere, possibly searchable in the FAA's online FSIMS database. I just can't find it!
Useful links:
E-CFRs and
FSIMS.
Anyway, I hope you find this info helpful.
Best,
Westhawk