A gas turbine engine comprising: a first spool (14,16) comprising a first compressor (18,24) rotationally driven by a first turbine (20,26) via a first shaft (22,28); a primary combustor; a diffuser having walls that diverge towards the primary combustor, wherein the first compressor (18,24), the diffuser, and the primary combustor are arranged in series to provide a compressed airflow discharged from the first compressor (18,24) to the primary combustor via the diffuser; and a second spool comprising a second compressor rotationally driven by a second turbine via a second shaft; a secondary combustor disposed downstream from the second compressor and upstream from the second turbine; wherein: the second compressor is fluidly coupled to the diffuser to receive at least a portion of the compressed airflow from the diffuser; the second compressor, the secondary combustor, and the second turbine are arranged in series to provide a compressed boost airflow discharged from the second compressor to the secondary combustor and a combusted boost airflow from the secondary combustor to the second turbine; and the second turbine is fluidly coupled to the diffuser to discharge an expanded airflow to the diffuser, characterized in that: the gas turbine further comprises a boost clutch selectively coupling the second spool to the first spool (14,16), wherein the second spool drives the first spool (14,16) when the boost clutch is engaged and the second spool rotates independently of the first spool (14,16) when the boost clutch is disengaged. a first spool (14,16) comprising a first compressor (18,24) rotationally driven by a first turbine (20,26) via a first shaft (22,28); a primary combustor; a diffuser having walls that diverge towards the primary combustor, wherein the first compressor (18,24), the diffuser, and the primary combustor are arranged in series to provide a compressed airflow discharged from the first compressor (18,24) to the primary combustor via the diffuser; and a second spool comprising a second compressor rotationally driven by a second turbine via a second shaft; a secondary combustor disposed downstream from the second compressor and upstream from the second turbine; wherein: the second compressor is fluidly coupled to the diffuser to receive at least a portion of the compressed airflow from the diffuser; the second compressor, the secondary combustor, and the second turbine are arranged in series to provide a compressed boost airflow discharged from the second compressor to the secondary combustor and a combusted boost airflow from the secondary combustor to the second turbine; and the second turbine is fluidly coupled to the diffuser to discharge an expanded airflow to the diffuser, the second compressor is fluidly coupled to the diffuser to receive at least a portion of the compressed airflow from the diffuser; the second compressor, the secondary combustor, and the second turbine are arranged in series to provide a compressed boost airflow discharged from the second compressor to the secondary combustor and a combusted boost airflow from the secondary combustor to the second turbine; and the second turbine is fluidly coupled to the diffuser to discharge an expanded airflow to the diffuser, characterized in that: the gas turbine further comprises a boost clutch selectively coupling the second spool to the first spool (14,16), wherein the second spool drives the first spool (14,16) when the boost clutch is engaged and the second spool rotates independently of the first spool (14,16) when the boost clutch is disengaged.