bi-wire speaker cables with single wire spkeakers

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

Don't know how Cardas terminates their cables but one of the nice features of my MIT cables is that you can just swap out the termination such that you have one pair spades and the other banana so they both fit nice and snug in a single set of binding posts.

be very careful if you "stack" the two spades onto one binding post. Cardas spades are pretty hefty and you will probably have to rotate one with respect to the other to get a connection. If you do that, make VERY SURE that the spades from one leg (+ or -) do not touch the other leg and that they are very snug so that they cannot shift. It would be very easy to short the outputs, in which case, if you are lucky, you will need to replace fuses; if unlucky, lots more. If I were doing that I would use electrical tape or some other insulator between the two legs.

I do the same and as mentioned stack both spades into the single post. To address the issue of them coming apart I tighten the post and stick an expired credit card or gift card in-between the Postive and Negative posts.