Hi Lennart,
I'm assuming you're using the licensed version of ReadyAPI.
Personally, I jump to Groovy scripting as I've been using Groovy with ReadyAPI for several years.
That said, there's usual more than one way to achieve the same thing.
There are lots of standard assertions you can apply to a particular request without resorting to Groovy or scripting. You can check for valid http statuses (200) or invalid (500), or depending on the use case, a valid http response may be a 500 if you're running some sort of negative request.
You can check for content existing, make sure a particular value is present and all sort of things. You can apply SLA type assertion that fail if the service doesn't return in, say, 200 millis.
It's worth investing a little time to play with these.
One of interest might be a 'Smart Assertion'. Start with your second request and add a smart assertion. Hopefully, Ready API will be able to pick out the value of interest. You can check values for equality, inequality, greater than, less than etc.etc.
In the valid value column, Ready API tries to anticipate what you want. You can overwrite the value to something else. But, what you can do here is clear the cell, then right-click in the cell and there is an context option called Get Data. This is probably one of the best Ready API features and is barely documented. Selecting Get Data allows you to navigate through you test hierarchy and select a value to check against. Using this, you could select the response from the first service call and check it against the second service call. Because you've used 'Get Data' the value is not hard-coded. ReadyAPI will dynamically pull this in.
Have a play with it. Because of the lack of key/value pairs, it might not help with this issue, but you will defiantely see how useful it is without resorting to scripting. If this doesn't help, comeback and I'll see I can help.
Regarding scripting in general. Yes, ReadyAPI is really powerful and straightforward to getting started with, but I'm pretty sure there will be times where a script help. IMHO, it is worth investing the time in learning some basic scripting, and this forum is an excellent place to get help.
It's worth noting that the majority of people that help on here are not affiliated with SmartBear or employed, we're just users of ReadyAPI who can help others. Bit like StackOverflow.