It's doable through the Item Scripting ( Edit Item Script ).
Just tell it to switch items when used on the correct page. For example, you want to have an Oil Item and a Vinegar Item that you mix together to make Salad Dressing Item.
Oil Item (#1) Default Page 100; Vinegar Item (#2) Default Page 200; Salad Dressing Item (#5) Default Page 500
On the Item Scripting for Oil, you use:
IF $PAGEID = 200 THEN
BEGIN
$ITEMSTATE1 := 0
$ITEMSTATE2 := 0
$ITEMSTATE5 := 1
$DEST := @P500
END
On the Item Scripting for Vinegar, you use:
IF $PAGEID = 100 THEN
BEGIN
$ITEMSTATE1 := 0
$ITEMSTATE2 := 0
$ITEMSTATE5 := 1
$DEST := @P500
END
This way, if you are on the oil or vinegar page and use the other item, it "mixes" them and takes you to the salad dressing page (removing the oil and vinegar items and adding the salad dressing item)...