Level secondary class?

[Niwashi]

Interesting. However, with thaumaturge class at 15 i already have surecast. Are we intended to level both classes in a job to 50?

It's only your primary class (Arcanist in the case of Summoners) that you'll want to continue all the way to 50 for the sake of that particular job, and playing the job advances that primary class anyway.
Secondary and tertiary classes for the job (Thaumaturge and Archer in this case) are only useful for the cross-class skills they bring in, so you want to level them to the level of their highest useful cross-class skill (so THM 26 and ARC 34).

Of course, in the process of doing that, you may well decide you like them enough to pursue their own jobs as well. If you like THM enough to become a Black Mage, or like ARC enough to become a Bard, then you'd continue with them the rest of the way to 50, but not for the sake of your Summoner job.

[Sagittarian]

Secondary and tertiary classes for the job (Thaumaturge and Archer in this case) are only useful for the cross-class skills they bring in, so you want to level them to the level of their highest useful cross-class skill (so THM 26 and ARC 34).

My only complaint with the way cross-class skills are set up is that you can't see in your current class which cross-class skills could be available if you leveled the other classes more. You basically have to look through all the skills in each of the other classes to see which ones are available to cross-class .. or you come to the forums and ask. It would be nice if they showed you all the available skills (assuming you have at least 1 level in the class) but grey out the ones that aren't available because of level - just the way they show abilities for your current class.

Also, for reference - http://ffxiv.consolegameswiki.com/wiki/Summoner