To your question, “how would you indicate its existence while laining?”
I have heard people try to express it in laining by pausing slightly between the “i” sound and the “oo” sound - i.e. reading it as if it were written:
תָּבִי אּ וּ
The problem is based on the problem of whether the vav-hachibur
that is marked with a shuruk and pronounced like "oo" in look is
really a kubutz or a shuruk.
Since kubutz is (according to R' Eliyahu Bachur) 'A movement between a shuruk and a chirik'
and a t'nuaa k'tana (small movement) the rule will apply that if it's not emphasised
a sh'va after ...
Short answer (as JoelK noted, see this one for more) -- disagreement among grammarians about cases like this. Listen to your favorite baal kriah read Exodus 7:29, about the frogs:
וּבְכָה וּבְעַמְּךָ, וּבְכָל-עֲבָדֶיךָ
All three of those "vet"s are in that same gray zone.
As it was taught to me -- you can't start a word with two sh'vas, as you'd ...