The eruv techumin system requires commitment. You gain one direction, lose the other. Two opposite eruvim = trying to have it both ways, which is forbidden.
Question 2
What is a conditional eruv?
Conditional eruvim accommodate uncertainty: before Shabbat, set a condition. "If the scholar comes east, my eruv is east; if west, my eruv is west." But you can't use both.
Question 3
When must conditions for an eruv be established?
All conditions must be established before Shabbat. Once Shabbat begins, your eruv choice is fixed based on the pre-set conditions.
Question 4
The underlying principle preventing two opposite eruvim is:
The trade-off principle: gaining distance in one direction means losing it in the other. Two opposite eruvim would eliminate this trade-off, which the system doesn't allow.
Question 5
If you set a conditional eruv and neither condition is met, what happens?
If neither condition is met, neither eruv takes effect. You remain at your default base — wherever you are at the start of Shabbat.