Generally* the housing authority that issued the Section 8 vouchers will have a "payment standard" by bedroom size. This payment standard is the maximum they will allow the rent to be based on the size of the apartment and it is based on the Fair Market Rents for your area. Fair Market Rents are set annually by the Dept. of Housing and Urban Development at the federal level, but based on the statistical information for each city/county/metro area. I don't know your exact area, but here's a link to the FMRs for Washington:
http://www.huduser.org/portal/datasets/fmr/fmrs/FY2013_code/2013state_summary.odn?inputname=STTLT*5399999999%2BWashington&selection_type=county&stname=Washington&statefp=53.0&year=2013&data=2013&incpath=C%3A\HUDUSER\wwwMain\datasets\fmr\fmrs\FY2013_Code&fmrtype=Final&path=C%3A\huduser\wwwdata\database
If the tenants aren't at the FMR and you want to raise the rent, you should be able to but you'll have to do it at the end of their Section 8 voucher lease period, which isn't always the same as their lease period with the landlord.
Before the sequestration, housing authorities could exceed the FMRs by a small percentage when "reasonable". I don't know how the sequestration has affected housing authorities in your area, but in mine, they are no longer exceeding FMR in any circumstances and are balking at paying the FMR at all.
Also, the housing authority should do annual inspections of the apartment which should help you with lease enforcement because they should be enforcing safe, sanitary housekeeping, unblocked/safe egress, and so on. Many landlords do not like the Section 8 inspections though because the housing authority is also checking the condition of the fixtures, appliances, flooring, etc. and often has a higher standard than the owner. They will require you to fix items to their standards or cease paying the subsidy, usually within a fairly short time frame (30-60 days). Since the tenants are already in place and you're considering living in one half yourself, the duplex is probably in good shape, so this may not be an issue for you, but I thought I'd mention it.
*I say "generally" because this is the way it's supposed to work. I have heard from owners/managers in many places where the housing authorities ignore the rules and do exactly what they want to do.