Your setup is not quite clear. Back in DDR days you had to identify interesting traffic to trigger the circuit connection. MLPPP, which I assume you're using, is an "always-on" connection, so it's not clear to me what do you mean "they come up".
If your requirement is to use two MLPPP links as failover, this should be resolved by the means of dynamic routing between PE and CE, where one circuit is chosen over the other to pass traffic due to better metric/cost.
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