As we know Jiren is a Over powered character. Even Beerus was mind blown when he released his Ki questioning whether this really the energy from just one warrior?! What if Jiren is a Fusion Character and he meditates in order to control the insane power within?
I understand why you would think that as fused characters tend to be very strong. However, it is certainly possible for even a regular character to be stronger than a fusion. For example, SSJB Goku was holding his own against a Kefla, Base Vegeta beats up SSJ3 Gotenks etc
That said, Jiren's level of power is not an unattainable feat. At least his base power was compared to a God of Destruction and all of the G.O.D are random individuals who have trained to attain their levels of strength. Even Goku achieves a level far beyond when he masters Ultra Instinct. Even Broly was able to reach a level comparable to a G.O.D.
Plus, being a fusion would completely contradict Jiren's character. Jiren has been indicated to be a loner who doesn't seek help from anyone and would much rather work by himself on account of his traumatic past. Fusion would imply he's dependent on someone else for his strength. So clearly, the character is not a fusion.