"Doubling the speed" implies that multiple connections over a single channel share bandwidth and not bandwidth time. In fact, the later and not the former are true and each connection gets a turn using the channel's total bandwidth.
As a result though the 2 connections/cards will operate over a single channel the time sharing of the bandwidth of that single channel may not equate to "double the speed".
The question then becomes: is there any speed increase or rather data throughput increase at all using a bonded interface over a single wifi channel?
The answer to that is it depends on how the data throughput is handled at the software level and here especially using a bonded interface and notably concerning signal loss packet drops (data integrity assurance) handling and bandwidth time sharing/switching.
(Also if there is something in the TCP/IP spec that for some reason narrows a channel's total bandwidth for each connection yet can widen it, perhaps by using a special mode, for some specific reason (which I doubt)).
Connectify (Windows), a multiple hardware interface merging application, does claim to increase data transfer speeds using, amongst other things, two wifi cards on the same machine over the same channel. How much faster, I don't know. The point is that unless they're lying, there is a leniency granted somewhere when using two wifi cards over a single channel.
I'll check some things out and edit this when I get some results using bond, james bond..
da ding dedela ding ding, da ding dedela ding ding, dun dun dunna, dunna dun..
PS: if it's possible you can request that R.Stallman write the bond code mods for free!
He is after all the father of free software.