How to get tox to test "named" test environments with different python versions?

e.g. Let's say I have following in tox.ini

[tox]
envlist = py27, py35, testenv2

[testenv]  
 # settings related to "default" testenv - includes deps, commands

[testenv:testenv2]  
 # settings related to testenv2 - includes deps, commands

Now when I run tox command, it invokes testenv commands with python 2.7 and 3.5 interpreter but testenv2 commands only with base python installed on the machine (in my case 2.7). How to get tox to also test a "named" (non-default) test environment like testenv2 to be tested with multiple python versions?