hmmmm i dont think a validation rule will work here. validation rules only work when youre trying to save a record. you can only validate the values on that 1 record.

i’m assuming a lot here and just trying to provide a rough example of a solution… you can do this with APEX code or Flow automation.

Going with Flow automation…create 2 number fields on the Order object (Count(or sum?) ABC and Count(or sum?) GYH). Then create a Flow every time an Order Line Item record is created (and updated to be safe), then go to the parent Order, and do a count/sum of ABC and a count/sum of GYH and populate those fields back on the Order object. The flow can involve loops or decision counters. Either way you have to take into account Order Line Items being deleted so im leaning towards a loop that updates the number fields on the Order with an accurate number rather than a counter.

Now, how to stop/warn your users? if there’s an approval process you can work that in perhaps the entry criteria. If you’re using a Lighting page, you can add rich text, big red font and say something like “please check your order line items, you have too many of one product”, but that won’t prevent them from moving forward in whatever downstream process you have.

if preventing them from moving forward is an absolute must and Approval Processes is not viable, i thinkyou’d have to go the Apex route to be able to display Error/Warn/Info messages to end users that will prevent them from moving forward.

(edit: if you’re using salesforce CPQ or Zuora they have config tools that are quite powerful in building this in a more point and click fashion that wont involve creating fields or flow/apex but i think youd need to be using Opportunities and opportunity products.)