You would have to raise enough energy for each spirit to be summoned. You would have to formulate your ritual so that it allows for you to summon each one individual and be able to hold them both for the length of time that you desire. I suspect such a task would wear you out and you would only get a few moments with them. This might be something that would be better done in a group, where half the group worked to summon one and the other half the other one.
Depends on the entities in question(for example loa, jinn, or other beings closer to this plane would be easier to materialize than angels, or something even more abstract), the magicians the question(stronger magician can materialize more, and more clairvoyant magician can see more), how solid you want the materialization to be(if you're fine just communicating with all those spirits, why do you need to see them all), how much energy there is to spare, and a myriad of other factors, down to the very location you're performing it(won't be the same if you're doing it in a haunted temple that's seen centuries of ritual work and an ordinary classroom)
Although it depends on the kind of spirit you're summoning, but keep in mind, the spirits you're summoning need to be in Harmony with each other otherwise it can create big trouble.
I mean like you cannot call a Demonic entity and an Angel together right?
Angels and demons tend to work fairly well together, though it depends on individual angels and individual demons. While I doubt you would have much luck working with Raphael and Azazel on the same operation(you can be friends with both, though), but other combinations, like Raphael-Marbas, Belial-Uriel, Sandalphon-Azazel and plenty more are perfect. If anything, this tends to make them even more effective as they work towards the same goal from two different perspectives and approaches, as opposed to tackling one problem with fifty different demons trying to do the same thing.
Granted, I have known many many individuals, belonging to all kinds of groups, from New Age, Christian mystics, to actual theistic satanists who completely endorse, believe in, and have experienced the "angels vs demons" war shindig, but in my reality, this has proven to be far from a universal case.
Granted, there are some demons and angels that don't get along well, at all, though that's a different matter altogether.