alternating spawns

This topic was touched upon, but did not exactly deal with what I'm trying to do-

I am making a CS:Source map which will have the teams alternating
spawns by round. I have set up two sets of t & ct spawns,
each partially covered by func_brush shapes to control when the players
will spawn at that site. eg: I have two sets of 20
player_terrorist entities partially covered by a func_brush- one set to
"start disabled - yes", and the other to "no". (This was covered
in another question on this site- the spawn was then controlled by a
button) What I need to do is have this setting alternate automatically
by round. I really have no idea how do make this happen, and I
have not been able to find anything elsewhere to assist me. Any
help would be greatly appreciated. (I suppose I could put a
button at one spawn that could alternate the settings, but I would
rather have it happen independently.)

The effect I'm going for would be similar to the gameplay of 1337sk337, if anyone is familiar with the map

Thank You!

(bump!)

If I were doing it, I would spawn people as shown in
this
tutorial. I don't know if Counter-Strike has a round change
trigger, but if it doesn't, you could improvise one by putting a
trigger over the info_terrorist spawns. Whenever a round ends,
turn off a set of trigger_teleports covering the spawn entities.
Turn on another set of triggers_teleports in the same location, all of
which point to the opposite side of the map.

Read that tutorial first, but if you need me to explain something, just ask.