Blackstone is shopping for up to $800 million of debt to finance its pending acquisition of two large office properties in downtown Seattle.The fund shop has agreed to purchase the 985,000-square-foot Wells Fargo Center and the 944,000-sf U.S. Bank Centre from Ivanhoe Cambridge for about $1.3 billion — one of the biggest property trades in Seattle’s history.Blackstone is requesting quotes on separate floating-rate mortgages at two...