First, there is no answer RAW that I have found.
Second, there is a possible cinematic solution.
We will start with page 118 in Essentials:
Interference is an Extra that lets the power’s dice directly disrupt
some other character’s action. On a successful roll each of your dice
becomes a gobble die that removes a die of that height or lower from
your target’s matching sets, before the target’s action takes effect.
If you have a 4d power with Interference and you roll 2x7 with it, you
can remove up to two dice of height 7 or less from your opponent’s set
or sets. Interference costs +3 per die.
Then page 127
Your Interference roll’s height must match or exceed the height of the
roll that you’re opposing. Unlike an ordinary opposed roll, however,
it does not need to match or beat the width of the roll you’re
The kinds of actions your power can oppose with Interference is always
up to the GM. Interference on a Defends Power Quality usually opposes
attack rolls; Interference on a Useful Power Quality usually opposes
noncombat rolls, depending on the nature and description of your power
and the action you’re trying to oppose. Interference on Attacks can
oppose just about any kind of action, but the GM is encouraged to play
up unintended consequences when you, say, distract someone with blasts
of supernova fire.
And Page 58 in regards to Resolution of Rolls
The character with the widest roll always acts first. If two sets are
equally wide, the taller roll goes first. All actions are resolved in
order of width.
So, we will assume in your case, that you have two people fighting, with two different powers.
Let's say we have a Blaster with a Harm (A;R) power, with Interference, at 4HD.
We also have a Teleporter with a Teleport (D;S) power, with Interference, at 4HD.
Inteference says it disrupts the action, equal or less in height, with no care about width. So, both of them go off, and both of them go off before the action. But we still need to resolve a Tie. They both have the same width. They both have the same height.
There is no RAW to what happens, that I have found. At this point, it is unfortunately up to the GM.
My ruling has been that in those cases, it's where we get those power vs power shots in cinema (The magic meeting in the middle, so to speak), or where two people punch each other at the exact same time, and maybe their fists meet, or they get thrown back, but are otherwise good to go.
In this case, the case of Interference, I would apply them to the other at the exact same time. Each one gobbles 4 dice. Three are needed to cancel the powers (This of course leaves each with 1 in case of multiple actions, or attacks from other places).
So, Blaster fires his ray, which excites the atoms around the area
enough that it temporarily disrupts the teleportation. But the
teleportation phasing left the Teleporter unharmed.
If you feel like you need to have a winner, however, you can have each roll a single die, and apply that height as the winner. That way it can change from action to action. This can show the example of cinema where the two powers meet, instead of cancelling the other out, one does eventually win.... but I prefer that that come about with an augment, or a change, or something else.