Firefighters responded to two separate fires early this morning in Kakaako and Nuuanu that could possibly be related, according to a fire department spokesman. The first fire was reported just after 2 a.m. at Stewart's Auto Service on Kawaiahao Street.  It burned two cars inside. Damage was estimated at $25,000. Fire Captain Terry Seelig says the fire appeared suspicious.   Fifteen minutes after the first fire began, fire crews were called to Pacific Heights Road, off Pali Highway, where a car was in flames.  That fire was also suspicious in nature, according to Seelig.   The Honolulu Fire Department is working with Honolulu police on the investigation.  They're asking if anyone saw anything, to come forward.