Their selection by the British Olympic Association (BOA) was a formality after they met the selection criteria earlier this month at the Synchronised Swimming Qualification Tournament at Beijing's National Aquatics Centre to finish first, one point clear of their nearest rivals, Katsiaryne Kulpo and Nastasiya Parfenova of Belarus.
Simon Clegg, the chief executive of the BOA, said: "I am particularly delighted to welcome Olivia and Jenna into Team GB for this summer's Olympic Games.
"This is the first time that Team GB has been represented in synchronised sming since Barcelona 1992 and shows the impact of increased lottery funding in the context of 2012.
"This shows the new strength and depth of emerging talent in Olympic sports across many disciplines and I wish them both well in their final preparations for Beijing."
Allison, 18, said: "I'm really happy to have been selected for the Olympics.
"It will be my first, and to be involved in such a prestigious event is an honour.
"This will be a great experience and will be a stepping stone as we look towards London 2012."
Randall, 19, said: "It's fantastic we are going to our first Olympics and will be a great experience for 2012.
"I am honoured to be selected as part of Team GB and look forward to returning to Beijing in August and showing the world what we can do."
Biz Price, the British Synchronised Swimming national performance director, said: "The duet are doing some great work and everything is going well for them.
"I'm very, very pleased with how Olivia and Jenna performed at the qualification tournament in Beijing in which they gave three solid performances.
"The scores Olivia and Jenna have been getting have been in the high 8's, and I will be looking at the programme and increasing the difficulty of the routines as we take the next step up ready for the Olympic Games."
photos Janos Schmidt