ZURICH (Reuters) - The Swiss government proposed on Friday tightening capital requirements for big banks that could prompt UBS and Credit Suisse to issue up to a combined 24 billion Swiss francs (18.41 billion pounds) worth of bail-in bonds.The draft proposal was put out for comment by the finance ministry until July 12."The additional requirements for funds ... amounts to some 24 billion Swiss francs for the two big banks," the draft proposal said, noting UBS and Credit Suisse would gradually..