Yes, you can have more than one anemone in a tank. But it really does depend on what anemones they are. Reefers have successfully kept different types of Bubble Tip anemones (BTA) in the same tank. In fact BTA’s often end up splitting therefore adding more anemones to your tank, whether you like it or not!
Having two different types of anemones can be very risky but not impossible. This should only really be attempted if you have a 5ft tank or larger though. This is because anemones release nematocyst which can irritate the other anemones in the tank. In a smaller tank it would be far more difficult for the tank to remove these nematocysts quickly enough resulting in possible death to your other tank inhabitants.
Another thing to consider is that you run the risk of them bumping into each other and stinging each other. The bigger the tank, the less chance there is of this happening. As well as having a large tank, you could ensure that you add carbon into the tank either in an activated carbon bag or a reactor. Regular water changes would also be beneficial as well as having spare RO water on hand in case of emergencies.
I personally would not want the stress of keeping two different anemones together in one tank but as mentioned above, it is possible. Some of the combinations I have seen are Rockflower anemones and Bubble Tip anemones, Sebae anemones and Bubble Tip anemones, Sebae and Mini Maxi Carpet anemones, Bubble Tip anemones and Rockflower anemones. These are not the only combinations but what all reefers had in common was a large tank. Large enough to keep the anemones far apart from each other.