Involved in the biosynthesis of branched-chain polyamines, which support the growth of thermophiles under high-temperature conditions. Catalyzes the sequential condensation of spermidine with the aminopropyl groups of decarboxylated S-adenosylmethionines to produce N(4)-bis(aminopropyl)spermidine via N(4)-aminopropylspermidine. Can also use spermine to produce N(4)-aminopropylspermine. 2 S-adenosyl 3-(methylsulfanyl)propylamine + spermidine = 2 H(+) + N(4)-bis(aminopropyl)spermidine + 2 S-methyl-5'-thioadenosine Amine and polyamine biosynthesis. Grows at 85 degrees Celsius with a slightly longer lag phase, but is unable to grow at 93 degrees Celsius. Belongs to the branched-chain polyamine synthase family.