How to resolve Lethal Tempo from Press the Attack
Simply put, allow Lethal Tempo to trigger against towers and epic (not large) monsters.
Given that both Runes deal approximately the same DPS late game and given that Press the Attack has several very clear advantages (amps your team's damage, no cooldown, three-hit proc is amazing for lane trades) it seems pretty clear that Lethal Tempo needs to have some advantage over Press the Attack.
We could just give it way more attack speed, but I don't think that's a very interesting solution. It just means people will choose whichever option is statistically stronger.
HOWEVER! If Lethal Tempo triggered against objectives, suddenly there's a very clear use-case for it. Do you want to team fight harder? Or do you want to secure objectives more quickly? This is a compelling choice where as 'do I want to deal damage or do I want to deal MORE damage?' is not a particularly compelling choice.