You can reterminate the cables at the speaker end, using new spades and joining the conductors at the spade. That would effectively convert your biwire cables to single-wire ones.
If you don't want to do the retermination, you can just connect two spades to a single binding post and tighten down very well. The risk is that the connection will loosen, so check it regularly. You can use one of Herbie's Binding Post Washers to help keep it snug. A shot of DeOxit Gold wouldn't hurt, and/or a silver paste like Mapleshade Silclear, to help secure the electrical connection.
Herbie's washers
If you don't want to do the retermination, you can just connect two spades to a single binding post and tighten down very well. The risk is that the connection will loosen, so check it regularly. You can use one of Herbie's Binding Post Washers to help keep it snug. A shot of DeOxit Gold wouldn't hurt, and/or a silver paste like Mapleshade Silclear, to help secure the electrical connection.
Herbie's washers