Logging second-in-command flight time. A person may log second-in-command time only for that flight time during which that person:

(1) Is qualified in accordance with the second-in-command requirements of §61.55, and occupies a crewmember station in an aircraft that requires more than one pilot by the aircraft's type certificate;

(2) Holds the appropriate category, class, and instrument rating (if an instrument rating is required for the flight) for the aircraft being flown, and more than one pilot is required under the type certification of the aircraft or the regulations under which the flight is being conducted;

Do a Barron/Seneca/etc require 2 pilots in their type certificate?

Not sure many situations exist where SIC can be logged in a light single pilot airplane. Curious to know what you are referring to in your 61.51 comment.

I think some Cape Air flights operate with a 2-pilot crew. This might be a case of SIC in a “light twin”. But this is, I think, 121 flying.

FWIW, I flew with many ex-Cape pilots in my previous life and without exception they were excellent pilots. We should recruit them.

I digress.

Places like Boutique and Plane Sense fly Pilatus PC-12's with 2 pilots. I'd imagine those in the right seat are allowed to fly SIC even though 2 pilots aren't required to fly a Pilatus.

The article below explains when you can and cannot log SIC in a single pilot plane (and gives references). Generally speaking, it has to be at a 91k/135 operator whose ops specs require it. Even if insurance dictates 2 pilots are needed, it still has to meet that FAA criteria or it isn’t loggable. There’s also a distinction on what can be logged by the FAA and what certain airline apps consider SIC time (and I think the article explains that as well). Bottom line is there are very few times when a light twin can have someone logging SIC.
https://www.boldmethod.com/learn-to-fly/regulations/your-guide-to-logging-sic-flight-time-in-single-pilot-airplanes/

The bold section is the determining factor. The type certificate does not need to require two pilots. If a second in command is required by regulation (regardless of aircraft type) the pilot may log SIC time in that aircraft. This is applicable under 135 for aircraft under 12,500lbs if the operator does not have an OpsSpec for "use of autopilot in lieu of second in command". I'm sure there are 121 equivalent scenarios like Cape Air.

This is rare but it does exist.

Ive always thought that the reason they had you list that time as SIC is because they want to know how many flight hours you have, but they really want to know how many PIC hours you have.

I guess they could change the inputs to include TT, PIC, SIC, but I think their current setup gets the job done. I dont think they are going to look at your app and ask why you listed SIC time in a baron when its a single pilot light twin. They just want to know how many hours you have and how many of those were you the signing PIC.

For the SWA app, I agree. There is no place to enter Dual Received. Entering any light single/twin time as SIC that does not meet the SWA PIC definition is the only way to make your times add up. This is separate from "logging" the time as SIC.