This is most likely a clocking setup issue. You are probably clocking both the
cards from their internal oscillators (clockMode = OTX_CLOCK_SOURCE_INTERNAL), in which case the boards
synchronize to different clocks and slips can occur. Slips are not always
catastrophic. They are simply an indication that the the transceiver is at the edge of its elastic buffer and it has
to either discard a frame or insert an extra frame to keep in sync.

To avoid slips, all
the cards should synchronize to one common clock.
With a two card setup, you can clock one of the cards from the internal
oscillator (OTX_CLOCK_SOURCE_INTERNAL) and let the other one to synchronize to an incoming span (OTX_CLOCK_SOURCE_LOCAL_0).

If you had the boards connected with H.100, then one of the boards
could synchronize to clocks sent over the H.100 bus.
If you have two boards looped together without an H.100 cable, you can try the
following setup:

Board A: OtxBrdSetClocks(hBoardHandle, OTX_CLOCK_MODE_LOCAL, OTX_CLOCK_SOURCE_INTERNAL);
Board B: OtxBrdSetClocks(hBoardHandle, OTX_CLOCK_MODE_LOCAL, OTX_CLOCK_SOURCE_LOCAL_0);

If you have two boards connected with an H.100 cable, then you could use the following:

Board A: OtxBrdSetClocks(hBoardHandle, OTX_CLOCK_MODE_H100, OTX_CLOCK_SOURCE_INTERNAL);
Board B: OtxBrdSetClocks(hBoardHandle, OTX_CLOCK_MODE_H100, OTX_CLOCK_SOURCE_A_CLOCKS);