Crossed Claisen condensation is a variation of Claisen condensation.

Claisen Condensation:

Crossed Claisen Condensation:

eg:

mechanism:
Step 1:  The alkoxide ion deprotanates the enolizable ester reversibly.

Step 2 and 3:  Enolate ion 1 undergoes a nucleophilic acyl substitution preferentially with the non-enolizable ester, which has the sterically less hindered and, therefore, more accessible carbonyl carbon, giving a  β-ketoester.

Step 4:  The alkoxide ion deprotonates the β-ketoester irreversibly.

Step 5:  The acid protonates enolate ion 2

see also Dieckmann condensation