In exploration mode, each player who wants to do
something beyond just traveling chooses an exploration
activity for their character. The most common activities
are Avoid Notice, Detect Magic, Hustle, and Search,
though there are many options available. While players
usually hew close to these default activities, there’s no
need for them to memorize the exploration activities and
use them exactly. Instead, allow each player to describe
what their character is doing. Then, as the GM, you can
determine which activity applies. This also means you
determine how an activity works if the character’s actions
differ from those on the list.
This would lead me to believe that you could in fact make the judgment that you did: If a character wants to both scout and search let them. 1/4 speed feels like a reasonable compromise for that.
Honestly Exploration Activities are more guidelines than hard and fast, "you can only do this," rules. I can easily see a character both Searching and Detecting Magic for instance. I probably wouldn't even require any additional speed penalties, instead maybe increasing the DC of any checks they make to show how their attention is split.
I put this in your duplicate post hours ago, but:
You can perform only one exploration activity unless you have an ability that states it can be done at the same time as other activities. Once the investigator comes in then it will technically be possible to maintain 4 exploration activities at once.
Avoid Notice - Legendary Sneak
Investigate - Ongoing Investigation
Search - Trapfinder
You also have your normal exploration activity that you can perform with that.