It is not currently possible to do this from Steam. Back in the day, I proposed to have both versions installed and selecting between then with a launch option. But the community didn't like this idea - reasoning was that it would be confusing for new users (Extended sounds "better" than Standard) and that Extended shouldn't really get that much exposure yet since it is not ready for average players.

So what can you do? My advice would be: install the Extended branch from Steam, since Steam will take care of daily updates for you. Then download Simutrans Standard latest stable version (122) from simutrans.com and put the executable in the Simutrans Steam folder (rename it to simutrans-standard so it doesn't conflict). Then you can add it to Steam as a non-Steam program and it should work. You will not have cloud save, tho.