keep upping the ante on each other , 
(ROOT (S (VP (VB keep) (S (VP (VBG upping) (NP (DT the) (NN ante)) (PP (IN on) (NP (DT each) (JJ other)))))) (, ,)))
(ROOT (S (VP (VB ) (S (VP (VBG ) (NP (DT ) (NN )) (PP (IN ) (NP (DT ) (JJ )))))) (, )))