Is feasible to load both Altera and Xilinx libraries to same Modelsim？

There should be a 'master' copy of modelsim.ini that is read-only in the install area.

When you call vmap the first time, Modelsim will detect that the master copy is read-only and create a local version for your project.

What this means however is that you end up with a modelsim.ini per simulation. I personally don't like this, so have setup environment variables to force Modelsim to use a single copy of modelsim.ini. The variables are discussed in an Appendix in the Modelsim manual.

For your tests, just deal with the fact that you need to have a modelsim.ini in the area that you are working. Make sure that the versions generated by Altera and Xilinx are somewhere else. Use vmap to populate your local modelsim.ini (or manually copy and paste the mappings).

Cheers,

Dave