1) Crossbar 2) Hilton hotel chain 
DoubleTree is an American hotel chain. It is part of Hilton Worldwide. Most DoubleTree hotels are independently owned and operated by franchisees, but some are owned and/or managed by Hilton Worldwide. DoubleTree serves a large warm chocolate chip cookie with walnuts to each guest at check-in. ...
• (n.) The bar, or crosspiece, of a carriage, to which the singletrees are attached.
Dou'ble·tree` noun The bar, or crosspiece, of a carriage, to which the singletrees are attached. 
noun a crossbar on a wagon or carriage to which two whiffletrees are attached in order to harness two horses abreast
a pivoted bar with a whiffletree attached to each end, used in harnessing two horses abreast. See illus. under