Hi Gav,
it's funny, last year I bought by e-bay a similar combination, a javanese keris/sheath with a (I think) sumatran hilt from Tridacna, here the link:
http://www.vikingsword.com/vb/showthread.php?t=8990
How can it happen that you find combinations like this? We have had discussions at this place many times. Collectors may have done this or dealer also. But I have seen this also on old pictures. So it's nearby that this mix maybe have been done long time ago in Indonesia or Peninsula by what reason however.
Do you have a provenance for this pieces in question?
A picture from up to the sheath-mouth would be nice to see how the blades fit inside will show if the scabbard is worked for the blade.