I have a sample of how I am performing 2 separate API's in a button. In this example I am performing an edit then an add in another table. The key is wrapping the next API in a double URLEncode. I included a style I have used that has a mouse over color change.
// Begin button style
var text bgcolor = "#808080";
var text bgcolorm = "#a1a1a1";
var text txtcolor = "white";
var text style = "style=\"text-decoration: none; box-shadow: 3px 3px 1px #888888;background:" & $bgcolor & "; color: " & $txtcolor & "; border-radius: 3px; padding: 5px 8px; display: inline-block; font-weight: normal;font: 700 24px/1 \"Calibri\", sans-serif; \" onMouseOver=\"this.style.backgroundColor='" & $bgcolorm & "'\";\" onMouseOut=\"this.style.backgroundColor='" & $bgcolor & "'\"; text-align: center; text-shadow:none;";
// End button style
var text URL = URLRoot() & "db/"& Dbid() & "?a=API_EditRecord&rid="& [Record ID#] &
"&_fid_1034=1"
& "&rdr="
& URLEncode(URLEncode(URLRoot() & "db/"& [_DBID_B] & "?a=API_AddRecord"
& "&_fid_6=" & [Store #]
& "&_fid_7=" & [New Store Integration Services]
& "&_fid_8=" & [Project ID]
& "&_fid_10=" & URLEncode([Task Number])
& "&_fid_42=1"));
"<a " & $style & " href=\"javascript:"
& "$.get('" & $URL
& "', function(){"
& "location.reload();"
& "});" & "void(0);\">Bill Integration Service</a>"