I have item A that I sell to Customer 1 and Customer 2. I get a forecast from marketing by customer/item. Customer 1 is a JIT manufacturer who I always want us to have inventory on hand, and customer 2 is a retailer who can wait 3 weeks for delivery.

I want to be able to build item A make-to-stock for Customer 1, and assemble-to-order for customer 2. Any ideas on how I can do this?

In Baan5, you can only set your time fences per item, not item/channel - so I have been struggling to come up with a solution on this.

Sorry, but that is not possible!

There is only 1 planitem A.

The use of channels is not meant for different planning methods.
It's (only) meant for (dis)aggregation of demand in MPS.

(or they must have changed it since I left last month....)

How about creating two phantom items, both consisting of the single item A? Then one can be set to MTS and the other to ATO.

Maybe?

Not really my department ;)