Pretty much any other European pear variety other than Seckel. Pineapple pear should work also. And yes, the trees must be flowering simultaneously.

Anjo (Beurre d' Anjou(), Conference, Harrow Delight, Magness, Shenandoah, Warren, Winter Nelis, and Hessle among others.

It's generally stated that there are 3 bloom periods for pears, but, for instance Cummins Nursery breaks it into 5 bloom periods.The above pears along with Bartlett are in bloom group B, the middle group. If you have two pears from the same bloom period they will pollinate each other. Except for the exception as already stated above. I live in zone 6a, so I wouldn't consider a tree from the early group,. I am working on a tree with now 8 different pears on the same tree, so next year I'm considering Bella Di Guigno in group A as I'd be willing to gamble one branch on a tree that might give me ripe pears in June every 5 years or so.