I know of one plug-in for WinAmp which does this. It's not the one I use because it actually causes glitches of static unless the two MP3s were encoded and split that specific way.
If there really is such a plug-in, then it must be one of the plug-ins for which source code is not included. I couldn't find one that did it; I actually had to modify the source code on one to get it to do true gapless playback with split MP3s. It was the mpg123 plug-in.
Michael Grant
12GB Green
080000266
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Michael Grant
12GB Green
080000266