The behavior you have described is absolutely proper.
You have evidently misunderstood what a Quota arrangement is intended to do. It will not split purchase orders or purchase reqs. It is only used to determine the source of each of the reqs you create.
So, if a new procurement proposal (in your case, a purchase req) is created, and you have an active quota arrangement, the system will analyze the data in the quota arrangement, and will use that information to determine what the source will be. There will be only one source for this PR.
The next purchase req you create will again analyze the data in the quota arrangement, including previously entered elements, and will re-determine which source should be used. In your 60/40 case, the typical result will be that the vendors will alternate each time you create a new purchase req, provided the quantities of the reqs are similar each time.
Please read SAP help for more info.http://help.sap.com/erp2005_ehp_06/helpdata/EN/75/ee115f55c811d189900000e8322d00/frameset.htm.