I was using whey protein powder before, can I can combine with XS Muscle Multiplier?

Both can be combined, but not at the same time. Protein supplementation can be taken as a meal replacement, in between meals as a snack and XS^TM Muscle Multiplier is recommended to be taken first thing in the morning or/and immediately after training for maximizing muscle protein synthesis/production.  

Protein provides satiety and more energy in the form of calories. It is up to the individual needs of the consumer depending on lifestyle, goals and preferences whether they choose to supplement with both or with one or the other.