I have a similar situation:
I have a duplex with tenants I inherited. Though I have owned the property for 2 years one tenant has been there for 9 years the other 7. I haven't raised rents yet but plan to do so this lease renewal in June. The rents are about $500 each where market is $550-600.
My plan is to increase each unit by $25 to edge them up while not increasing to the point that will drive them out.
I may increase an additional $25 in a year or two if neither tenant moves out. This property cash flows about 8K/year with current rents so it still performs well. I just don't want to be in the business of owning property with very low rents. So maintaining the balance of rents to market will be a struggle with this particular property so long as these tenants are still there. The cost of raising rents and having them move out if far larger than keeping them there in my estimation because I will have to update both units to some extent.