Nope.
The second summon will always displace the first. That's a balance thing (and discussing any possible workarounds would be a violation of the forum's rules).
A summoner mage is pretty solid with only 1 summon. In really tough fights, you might find it useful to summon a 2nd create right before the 1st one dies. That way there's a minimal gap in protection.
There's pros and cons of going with 1 or 2 major summons. Getting all three major summons uses up a lot of skill points but has its benefits, too. My go-to summon mage is Summoner + Staff. Gives a nice balance. That also means your damage output is high enough, the golem's main purpose is a meat shield. The Iron Golem is the go-to for that role.
While the Iron Golem gets a lot of kudos, don't overlook the Fire Golem. Due to the way the certain areas of the game are designed, the fire golem edges out the other golems as the single overall most useful golem. Fire heals the Fire Golem, and the Fire Golem can kill trolls.
But the Iron Golem is a solid, all purpose summon beast. Arguably the best golem for general dungeon runs, but looses some of its usefulness in a few spots, which allows the Fire Golem to nudge it out.
Ice Golem might be the best to use against the Elemental Lord (there's some debate) and Ice Dragons, but the reality is that a scroll or two will cover your ice golem needs in the early to mid-game, and after that, your Iron Golem will be strong enough to make any difference rather tiny.
There are some really nifty Ice Golem builds so its not like the ice golem is a waste. Just a different build with different strengths.
So, yeah you don't really need two summons.
