Yes it is extremely safe to combine methadone with fentanyl because you cannot overdose on either drug by itself therefore combined it's even safer.
You can't overdose on methadone alone? Or fentanyl alone? Seems like alot of people die from both

You can see the phenomenon illustrated here:

Methadone is quite rare relative to other prescription opioids, yet it kills a disproportionate amount of people considering its comparative scarcity. For instance, as shown above, in 2005, all the natural and semi-synthetic opioids prescribed combined (ie, all scripts for morphine, hydrocodone, oxycodone, hydromorphone, oxymorphone etc) killed around 5,500, while methadone killed around 4,500, despite methadone being far rarer.
Its perhaps even more clear here:

Methadone of course will raise your tolerance to fentanyl, and vice versa, but the combo can definitely result in fatal OD.
That said, I used a ton of fentanyl while on methadone. Mainly because it was the only thing strong enough to pierce through the 380mg/day of methadone I was on.
Perhaps all the fentanyl that is going around these might days might wind up leading to a reduction in the number of methadone overdoses since all that fentanyl that is being consumed has raised people's tolerances to the point where methadone overdoses don't happen as easily.