Concatenate boost::dynamic_bitset or std::bitset

Question

what is the best way to concatenate 2 bitsets?

For example i've got

boost::dynamic_bitset<> test1( std::string("1111") );
boost::dynamic_bitset<> test2( std::string("00") );

they should be concatenated into a thrid Bitset test3 which then holds

111100 

Solutions should use boost::dynamic_bitset. If the solution works with std::bitset, it would be nice too. There should be a focus on performance when concatenating the bits.

UPDATE: I've compared both methods (stringmethod from me and Neil and shiftmethod from messenger) and the stringmethod was a lot faster (factor 10++). Code here: http://pastebin.com/HfpfYfy8

I hope Pastebin is ok for posting long code-listings. If there is a better way please contact me.