、。
Doing two clubs at once must be tough, but good luck.
Literal
Club [gen-の] double-hold, tough [quotative-だと] think-[but-けど] good-luck [よ].
かけもち ('doubling up / holding multiple') is a productive noun from かける + 持つ — commonly used for holding multiple jobs, clubs, roles, or loans. 部活のかけもち ('doing two clubs') shows someone taking on dual club commitments. 頑張れ is the plain-form imperative of 頑張る ('to do one's best'), the standard encouragement in Japanese. よ softens the imperative into friendly support.