Is it scriptural for a preacher to accept support from two churches at the same time? e.g. from the church where he is located and the other from the church which he helped established (currently preaching in both churches).
"I robbed other churches, taking wages from them to minister to you. And when I was present with you, and in need, I was a burden to no one, for what I lacked the brethren who came from Macedonia supplied. And in everything I kept myself from being burdensome to you, and so I will keep myself" (II Corinthians 11:8-9).
Paul was supported by multiple churches while he was working in Corinth. This doesn't mean he was drawing multiple salaries. He was being provided enough to live on, but the source for this funding happened to come from several churches. When there wasn't enough, Paul willingly work to support himself and those who traveled with him. "I have coveted no one's silver or gold or apparel. Yes, you yourselves know that these hands have provided for my necessities, and for those who were with me" (Acts 20:33-34).
What you don't find in the Scriptures is a man preaching at multiple congregations at the same time. While there might be temporary needs because of a lack of preachers in the area, this should not be the normal mode of operation. The main reason is that the preacher no longer sees himself as a member of a particular congregation. There is also a tendency to create an office above the local church, where a preacher is operating multiple churches. See: Must a Preacher Move? for more information on why this is a problem.