Doesn't it usually work the other way round? Multiple purchase requisitions (from various requestors in the organization) for the same vendor can be pulled together into one PO. At least that's how I understand the process from an SAP environment.

Purchase Orders that are generated are always generated per supplier. So if you have 1 request (from a Sale Order) for 3 bolts from vendor A and 6 nuts from vendor B, there will be 2 purchase orders. 

If you want to combine all the purchase orders from different requests (for example, 1 request from a Sale Order and 1 from a minimum stock rule) you can do the following:

1. Go to the list view of the purchase orders
2. Select all the orders you want to merge by checking the box in front of them
3. Click on the "More" button on the top of the list
4. Select the option "Merge purchase orders"

This should give you a purchase order per supplier/vendor and cancel the requests that have been merged.