Southeast Asian heads of government will spend less than eight hours together in working sessions at a shortened regional summit in Manila this month. Asian diplomats said that worries about security in Manila, where coup attempts are common, led to shortening of the meeting. A program released by the Philippine Foreign Ministry shows the six-nation summit reduced to two days, Dec. 14-15, from the original three. Including preliminary meetings, the gathering will cover eight days rather than the original 11. Attending the meeting of the Assn. of Southeast Asian Nations will be leaders of Brunei, Indonesia, Malaysia, Philippines, Singapore and Thailand.