And police believe that the two incidents are connected.
The robberies took place between 2pm and 4pm on Tuesday (April 5).
At around 2.15pm, a 14-year-old boy was allegedly threatened into handing over his mobile phone, in a loading bay behind Waterstones.
Then, at around 3.30pm, a 16-year-old boy was allegedly threatened into handing over his wallet.
Fortunately, no one was hurt during the incidents.
PC Daniel Noyes, from the Dacorum Local Crime Unit, said: “Our enquiries are continuing at this time and an arrest has been made.
“As part of our ongoing investigation, we’d be keen to hear from anyone who was in the area at the time and saw the incidents take place, or anyone who witnessed a male acting suspiciously. If you can help, please get in touch.”
A 15-year-old boy from Hemel Hempstead has been arrested on suspicion of robbery, possession of a bladed article and possession of Class A. He has been released on police bail while enquiries continue.
Anyone with information is asked to email [email protected]
You can also report information online or call the non-emergency number 101.