I believe this is syntactically correct:
{% comment %}
pseudocode:
for line_item in order.line_items:
if line_item.sku exists:
products_and_quantities = split line_item.sku with '+'
for product_and_quantity in products_and_quantities:
new_order = split product_and_quantity with 'x'
quantity = new_order[0]
product = new_order[1]
if product.sku exists in products:
overwrite order: add quantity of product to order_list
{% endcomment %}
{% for line_item in order.line_items %}
{% if line_item.sku %}
{% assign sku = line_item.sku %}
{% capture products_and_quantities %}
{{ sku | split: "+" }}
{% endcapture %}
{% for product_and_quantity in products_and_quantities %}
{% capture new_order %}
{{ product_and_quantity | split: "x" }}
{% endcapture %}
{% capture quantity %}
{{ new_order | first }}
{% endcapture %}
{% capture product %}
{{ new_order | last }}
{% endcapture %}
{% endfor %}
{% endif %}
{% endfor %}