No, one of the beauties of ThumbsPlus, I think, is that it shows you all the files of whatever types and filenames that you have chosen to see. In my opinion, a feature like you describe from Lightroom where one thumbnail can represent multiple files would only create confusion.
Anyway, you don't have to go into the filetype definitions to hide one particular type. You can just type *.jpg in the filename mask box above the thumbnail listing (to the right of the funnel). Or use "*.jpg;*.nef" when you want to see both. Whatever you enter will be remembered for future re-use in the pulldown thingy. To reset it to 'selected types' you can click on the funnel.
