Hellllooo!
Alrighty, you definitely can use heroin while also on methadone, it's risky, as all this stuff always is. But yes it will work.
The main issue I can think of is drug testing. Rules are different in different places for whether you have to be giving clean urines to stay on methadone, So the main issue I'd be concerned about is if there were any issue if you came up positive in a later drug test. It prooobably would be ok, it's just that the rules vary, I know what the rules are like where I live, and here I don't think it'd be a problem, but if you're using tar, you definitely don't live where I live.
It also varies how fast they will increase your methadone dose, as well as what they will start you on. Here if you're a heavy user they generally start you on 30-40mg.
You might well have some withdrawal in the first few days, but if you get your methadone early in the morning (as a lot of people who have jobs to get to need to) then it probably won't be too bad.
It takes a few days for the methadone to build up, regardless of what dose you start on, so by day 3 30mg will be more effective than day 1, but it's likely you'll keep increasing up till when you tell them you're comfortable, and not sick when you wake up. For me I'm on 80mg.
Main thing is to be able to get through that first week, it shouldn't take too long to get up to a dose where you're feeling a lot better, probably inside of the first couple weeks. But you gotta keep going for those couple weeks, cause if you miss multiple doses in a row they often won't give you your full dose, which then of course increases the risk of you continuing to use.
I was definitely still using though for the first week or so when I got on methadone. Actually I've been inducted into a methadone program I think 3 times and was using the first weeks each time. First 2 times didn't really stick, but I was very serious by the 3rd and got stable.
The heroin will most likely still work and be euphoric (or about as euphoric as normal) in the first few days since you'll likely not be on that much (for your tolerance anyway) and it'll be building up in your system, but eventually once you're on enough, it's likely you won't feel the need for heroin and will probably feel like you can tell that it wouldn't be very good if you had it (at least that's how it felt to me). I'd be cautious to not get on too much more than you need to though cause it can be quite sedating, particularly in the first few months. You wanna feel well pretty much all the time, but still functional.
I wish you luck

If you have any other questions please feel free to ask.