Most GPs won't have a problem with 2 complaints, provided it's not going to significantly affect subsequent appointments (contrary to Nix's experience, and tbh mate I'd write a letter to the practice manager, particularly if you tried to make another appointment to discuss the second prob and couldn't get one, and/or if the GP has an attitude problem)
Most GP surgeries will have a specific policy, but there is no national guidance, and usually depends on the particular doctor - for instance I would always block 10 mins in every 60-90, to allow me to overrun a bit on appointments for this very reason as I think it is medicolegally indefensible to ignore a problem a patient raises (especially as repeated studies show that, particularly with men, the 'incidental' or 'and another thing doc...' concerns of patients tend to be the more serious/concerning complaints).
If there is no clear policy, usually just use common sense, remember this will only leave the GP 5 mins per problem (from calling you in, taking the history, examination, making the diagnosis, deciding on treatment, writing it up, +/- referrals etc)